Most feed stores carry Corid, they may have the liquid in the poultry section, if not look for the powder in the cattle supplies section. There is no egg withdrawal on Corid.
Since they have already been on the ground they will have built some resistance, the problem being that there are at least 9 different strains of the coccidia protozoa. So if they come to your place and encounter strains new to them they are again susceptible to coccidiosis while they develop...
What you need to do, after normal quarantine, is fence off an area for them where they can be alongside your existing flock but separated by a fence. Chickens hate newcomers and they can be seriously brutal. Your new birds don't need to getting beaten up by the others while they are also...